Molded-in composite bushings

ABSTRACT

This invention is directed toward a fiber reinforced resin matrix composite article having fiber reinforced resin matrix bushings. These bushings are formed by tightly wrapping the fibers to form a plug having the fiber orientation from about ±20° to about ±60° to the central axis around which the fibers are wrapped. The plug is then snugly inserted into a hole which has been cut in the composite article such that the central axis of the plug is perpendicular to the fibers in the composite article. The entire composite article containing the plug is then cured and the plug is then drilled out forming the bushing.

This is a division of application Ser. No. 499,699 filed on May 31,1983.

DESCRIPTION

1. Technical Field

The field of art to which this invention pertains is compositematerials, and particularly composite attachment means.

2. Background Art

Recently, there has been a tremendous surge in the interest in replacingtraditional metal components with composite materials. There are anumber of reasons why this is so, three of which are quite important.

First, composite components are considerably lighter than their metalcounterparts. This particular feature is of particular interest to theautomobile and aircraft industries where weight reduction means loweroperating costs such as fuel savings.

Secondly, many of the metals which are being replaced are consideredcritical, especially if they are useful in aircraft. The criticaldefinition is used to define a material which is in short supply orwhose supply is dependent on foreign sources and for which there is nota readily available alternative. The particular issue of dependency wasdramatically pointed out in the oil embargo of the early 1970's.

The third reason is that of integrity of the metal components or lack ofdamage tolerance. Metal has a tendency to develop cracks when itfatigues. There is no inherent way in which these cracks can beprevented from propagating, resulting in a complete failure of thatcomponent.

It has been shown that different combinations of fibers and matrixes,having preselected fiber orientation can be developed which go a longway to overcoming each one of the metal's shortcomings.

Recently, composites have been constructed which are so reliable thatthey can be used as damage tolerant components on aircraft, i.e. rotorhubs, and blades for helicopters.

However, there is a problem in using composite structures as componentparts. The problem is how to effectively fasten or connect the compositecomponents to the other components without weakening the compositecomponents. A typical approach to connecting a composite component tosomething is to form the composite structure and then drill the requiredholes through the composite. A bolt, screw or other conventionalattaching means is then passed through the hole of the compositestructure and the adjoining structure. This technique is unsatisfactoryfor many applications because the area surrounding the hole is subjectto galling and delamination from both the drilling process and theaction of the screw, bolt or other fastening device. An alternateapproach is to drill the hole on the cured composite and bond aprotective metal bushing to line the hole to prevent galling. However,the use of the metal or any other secondarily bonded bushing, results intwo drilling operations plus a bonding operation. The initial drillingof the composite would still result in the same delamination whichoccurred as before. In addition, it has proven difficult to bond themetal bushing so that the bond does not break when the bushing isdrilled to the proper dimensions, resulting in the bushing spinning outof the hole, which can result in damage to the main structure.

Therefore, what is needed in this art is a simple procedure forfabricating composite components having holes which resist delaminationand galling.

DISCLOSURE OF INVENTION

The invention is directed toward a method of making a composite articlehaving at least one fiber reinforced resin matrix composite bushing.

The following commonly assigned U.S. patent applications are hereinincorporated by reference, U.S. Ser. No. 477,696 entitled "A FiberReinforced/Epoxy Matrix Composite Helicopter Rotor Main Hub Plate"; U.S.Ser. No. 477,701 entitled "Fiber Reinforced/Epoxy Matrix CompositeHelicopter Rotor Yoke"; U.S. Ser. No. 477,708 entitled "CompositeHelicopter Rotor Hub"; and U.S. Ser. No. 477,709 entitled "FiberReinforced Epoxy Matrix Composite Helicopter Rotor Torque Tube" allfiled on Mar. 22, 1983.

The method involves preparing each resin impregnated fiber containinglayer of the composite article by cutting it to the proper articleshape. These fiber layers may be of the raw fibers on which the resin islater applied or the fiber layer may be pre-impregnated with the resinmatrix. Each layer has the holes, which are to form the connectingpoints, punched into it at the appropriate position. The requisitenumber of these layers are then stacked one on each other with the holesaligned, within the curing fixture. A plug of uncured resin impregnatedfiber is then formed by tightly wrapping the resin impregnated fiberaround a central axis such that the orientation of the fibers is about±20° to about ±60° to this central axis (which would be oriented about±70° to about ±30° to the fibers in the composite article). This plug,which is wound to the dimensions of the holes in the composite, is thenfitted snugly inside the precut holes such that the central axis isperpendicular to the fiber orientation of the composite lay-up. Theentire uncured composite article, along with the plugs, is then cured.The bushings are then formed by drilling holes to the proper dimensionsin the plugs with conventional drilling equipment and techniques. It iscritical that after the bushing has been formed, that the internaldiameter of the bushing contain at least one layer of fibers which hasan orientation of about ±20° to about ±60° to a vertical axis to theplane of composite fiber materials. It is this particular fiberorientation which prevents the delamination and galling of thesebushings.

It has been found that during the curing cycle that the matrix and tosome extent the fibers are meshed with the basic composite materialforming a very positive mechanical lock between the bushing and the maincomposite article.

Another aspect of the invention is a fiber reinforced resin matrixbushing, wherein the orientation of the fibers in the bushing are about±20° to about ±60° to the central axis of the bushing around which it iswound.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the preferredembodiment and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows punched holes in uncured composite lay-up.

FIG. 2 shows preparation of a ±45° plug using woven prepreg.

FIG. 3 shows preparation of a ±45° plug using filament winding ofprepreg.

FIG. 4 shows preparation of a plug having ±45° fibers internally and 90°fibers externally.

FIG. 5 shows cross section of a bushing in a cured composite article.

BEST MODE FOR CARRYING OUT THE INVENTION

During this discussion, it is assumed that someone skilled in the artknows and understands the common practices involved in preparing fiberreinforced resin matrix composites.

The present invention may be practiced with any fiber reinforced resincomposite materials in which the fibers are capable of being tightlywound around a central axis. The fibers and the matrix used in formingthe bushing may be the same as that which are used in the primarycomposite article, but need not be so limited. Any combination of fiberreinforcement and resin which is compatible with the primary compositematerials may be used. The primary concerns in determining compatibilityare, among other things, curing temperature of the matrix, chemicalcompatibility of the resin system matrix with the primary matrix as wellas the physical properties of the bushing components, i.e. coefficientof thermal expansion, etc.

The primary composite article was formed using a combination of highstrength graphite having typical properties such as 480 KSI tensilestrength and nominal tensile modulus of 33×10⁶ PSI interleaved withS-type fiberglass cloth, the fiber orientation being unidirectional.Each layer was cut into a 6 in.×6 in. square and two 1 in. holes werepunched into it. The layers were then stacked upon each other in thecuring fixture to form the primary lay-up as shown in FIG. 1. Plugs ofjust high strength graphite fiber/resin matrix were formed by windingthe pre-impregnated graphite cloth tightly upon itself. The cloth waswound such that the fiber orientation was about ±45° to a central axisaround which the cloth was wrapped, FIG. 2. The winding continued untileach plug was 1 inch in diameter. This required 36 wraps of the cloth.The plugs were then inserted into the precut holes in the compositelay-up and the entire article was cured at 325° F. for two hours.

After the article had cured, it was removed from the fixture and theplugs were drilled out using a conventional 0.5 in. drill to form thebushings, FIG. 5.

Other techniques may be used to form the plug such as winding a tow offibers around the central axis as in FIG. 3. A small, removable mandrelshould be used to start the winding.

An alternate configuration of these composite bushings is to prepare theplug as described and then wrap at least one layer of the resinimpregnated fiber around the external diameter of the plug such that itsfiber orientation is about 90° to the central axis of the bushing, FIG.4. This 90° layer allows for less disruption of the fiber orientation atthe interface of the plug and the primary composite article. It shouldbe understood that this is an additional layer and not a replacement forthe ±20° to ±60° oriented fibers.

Although the orientation of the fibers has been disclosed as 45° with anoptional 90° outer wrap, this technique may be practiced with otherorientations as well. However, the internal wraps forming the bushingshould be limited to about ±20° to about ±60° in order to preventdelamination from occurring.

Bushings made using this method have excellent stability as they aretotally integrated into the composite article. They are easy to form andeliminate a number of operational steps required in implanting bushingsusing other techniques. These integrated composite bushings form a verypositive mechanical lock between the bushing and the composite articlehaving a high resistance to galling and delaminating due to the actionof the connecting unit.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritand scope of this novel concept as defined by the following claims.

We claim:
 1. A method of making a fiber reinforced resin matrixcomposite article having a fiber reinforced resin matrix bushingcomprising:preparing an uncured, resin impregnated fiber reinforcedcomposite article having at least one connecting hole formed therein,tightly wrapping uncured resin impregnated fibers around a central axisforming a plug, until the diameter of the plug is substantially the sameas that of the formed hole in the uncured composite article, the plug isthen inserted into the formed hole of the composite such that thecentral axis around which the plug is wrapped is substantiallyperpendicular to the composite article fiber orientation, the compositearticle containing the plug is then cured, and an attachment hole of theproper dimensions is then drilled into the plug forming the bushing,said bushing having at least one layer of fibers which are orientedabout ±20° to about ±60° to the central axis of the bushing.
 2. Themethod of claim 1 wherein the wrapping of the plug includes wrapping atleast one outer layer of fibers oriented about 90° to the central axisonto the about ±20° to about ±60° oriented fibers.